It is widely practiced to use an ultrasonic flaw detecting apparatus to evaluate a flaw state such as a void and a crack of a joint region in a structure or a component that is a test object.
An ultrasonic wave transmitted from a piezoelectric element to such a test object is reflected chiefly by a surface, a flaw, and a bottom of the object and the piezoelectric element receives echo signals from them. Assuming that a moment a surface echo has been received is a starting point and a moment a bottom echo is received is an ending point, an echo signal that is received within a range from the starting point to the ending point is treated as a flaw echo. Then, information such as a position and a size of the flaw in the test object is visualized by detecting intensity and a timing of the flaw echo (for example, see Patent Document 1 (Japanese Patent Laid-Open No. 2003-121426)).
It is known that in an ultrasonic flaw detecting apparatus, spatial resolution of a detected flaw decreases with increasing depth of a test object. Therefore, conventional ultrasonic flaw detecting apparatuses have a problem that if a flaw is at or near a bottom or a corner of a test object, visibility of a visualized flaw decreases because multiple echoes interfere with a flaw echo (see FIG. 6(C)).
The present invention has been made in view of such circumstances, and provided an ultrasonic flaw detecting technique that can accurately and rapidly detect a flaw at or near a bottom or a corner of a test object.